1. Field of the Invention
This invention relates to a porous film of polytetrafluoroethylene having an accurately controlled pore diameter and a superior pore diameter distribution, and to a process for preparing the film.
2. Description of the Prior Art
As is well known, polytetrafluoroethylene resins have superior chemical resistance, heat resistance and electric insulation and also self-lubricating properties and non-tackiness, and find a wide range of utility both in industrial fields and in every day life. On the other hand, these superior properties are indicative of the difficulty of processing these resins. Accordingly, extensive research and development work has been conducted. One noteworthy method that emerged from such work involves fabricating an unsintered tetrafluoroethylene resin mixture containing a liquid lubricant by extrusion or rolling, etc., and then while being stretched in at least one direction in the unsintered state, sintering the mixture to at least about 327.degree. C to form a porous structure. This method can provide a porous film having a relatively uniform porous structure and superior strength, but is insufficient for obtaining an accurate control of the pore diameter and the desired pore diameter distribution.